The desire to maintain a youthful appearance by preventing or reducing wrinkles in the skin is an important issue in human society. Many techniques have been designed to address the above issue. One of the techniques, e.g., known from the published international patent application WO 2008/001284 A2, is to create a focal spot in a dermis layer of the skin to be treated. Said WO application discloses a skin treatment device with a laser source and focusing optics. The device emits a laser beam. The power of the laser is selected such that Laser Induced Optical Breakdown (LIOB) affects the skin in order to stimulate re-growth of skin tissue and reduces wrinkles. This LIOB is based on strong non-linear absorption of the laser light by the skin tissue, which occurs above a certain threshold value for the power density of the laser light. This strong absorption causes a very localized plasma that is able to damage or even remove tissue at the location of said plasma. This is caused by secondary, primarily mechanical, effects such as rapid expansion of the generated plasma. This effect is very local, because below the threshold, there is zero or very little linear and non-linear absorption, while above the threshold a plasma is generated, which absorbs the radiation even more strongly.
More details on LIOB may be found in the article “Minimally invasive non-thermal laser technology using laser-induced optical breakdown for skin rejuvenation”, Habbema, Verhagen, van Hal, Liu and Varghese; J. Biophotonics 5, No. 2, 194-199 (2012); DOI 10.1002/jbio.201100083. As discussed in this article, LIOB occurs when the irradiance is sufficiently high to produce a critical free-electron density of about 1021 cm−3.
An increasing number of these skin treatment devices are being provided for use by consumers instead of by medical professionals. These devices are non-invasive —they create an effect beneath the surface of the skin without having to physically penetrate the epidermis. However, such home use raises new concerns, such as safety and treatment efficacy. This is particularly important when the light source is a laser, and incorrect operation can result in scarring or burning of the skin where the laser light passes through the skin layers.
Damage to the epidermis, for example, is highly undesirable because this may lead to complications and health risks to the person being treated, as well as social downtime. If superficial lesions are created above the dermis, petechiae (micro-bleeding) may occur due to the micro-rupturing of capillaries, resulting in reduced efficacy and an increase in side effects. The formation of new collagen for the purpose of skin rejuvenation will occur if lesions are created in the collagen. At large focusing depths, the intensity in the focus may not reach the threshold required for optical breakdown and therefore the treatment may not produce the desired rejuvenation effects. The effectiveness of optical breakdown for skin rejuvenation depends on several factors such as optical and structural properties of the skin, laser light intensity in the focus, optical coupling etc. There is an upper limit for the photomechanical effect that can be used to induce growth of new collagen fibers for rejuvenation effects and treatment beyond this limit is not effective and additionally subjects the user to the dangers of over-use. Undertreatment is not preferred either because it may not produce the desired tightening effects.